In the field of thermosetting coating materials including those for automobiles, cross-liked coating materials employing amino resins such as melamine resins as a hardening agent have typically been used for a long time due to the excellent appearance and weatherability of the resulting coating films, as well as stability and economical efficiency. Recently, however, it has been revealed that coating films having melamine type cross-linking are suffered from depressed water spots (etching) which cannot be washed off with water, which has become a serious problem. Further, as the number of automobiles coated with dark-hued coating materials has been increasing, scratches made with brushes in washing have also been a remarkable problem. Accordingly, coating materials for automobiles having superior abrasion resistance against scratches are strongly demanded.
In order to overcome these problems, compositions having various hardening systems have been proposed. For example, Japanese Laid-open Patent Application No. 51-114429, EP-A-29 595, U.S. Pat. Nos. 4,371,667, 4,650,718, 4,618,811, 4,703,101, and 4,764,430 disclose compositions including a combination of a carboxyl group and an epoxy group.
U.S. Pat. No. 3,505,297, Japanese Laid-open Patent Application Nos. 60-88038 and 2-115238 disclose compositions including a combination of a carboxyl group and an oxazoline group.
These thermosetting compositions give cured products having excellent chemical and physical properties as well as weatherability, and are widely utilized in the fields of coating materials, inks, adhesives, and molded plastics.
However, reactivity between the carboxyl group and the above-mentioned reactive functional groups is so high that compositions containing both a compound having a carboxyl group and a compound having such reactive functional groups are often gelled during storage and the period suitable for the service of the compositions is short.
Compounds having a carboxyl group used for the conventional thermosetting compositions have drawbacks that their solubility in generally used organic solvents is low and that their compatibility with the compounds having functional groups reactive with a carboxyl group is inferior, since the carboxyl group has a strong hydrogen bond. Thus, when the thermosetting compositions containing such compounds are used as a top coating material, it is difficult to prepare high solid coating materials, and appearance of the finished coating is inferior.
Japanese Laid-open Patent Application No. 3-172318 discloses a coating film produced by curing a coating composition containing a blocked hydroxyl group, an acid anhydride group, an epoxy group, and a hydrolytic silyl group. This coating film has excellent acid resistance and abrasion resistance and thus is useful, but the storage stability of the coating material is not sufficiently high.